1. Field of the Invention
The present invention relates to a servo device for a spindle motor suitable for a recording/reproducing apparatus for recording, reproducing and erasing an information on a magnetic disk, and more particularly to a servo device capable of performing a precise rotational speed control and speed superintendence by eliminating an error which results from a magnetizing state of a magnet which constitutes a rotator of the spindle motor.
2. Description of the Related Art
In general, a spindle motor is used for rotatively deriving a memory carrier of disk type in a magnet disk apparatus.
For example, the number of rotations is from about 2,000 rpm to about 4,000 rpm in the magnetic disk apparatus and a servo mechanism for controlling the rotational speed of the motor is disposed to keep the number of rotations constant.
FIG. 10 shows a side view of main constructional elements around the spindle motor in the magnetic disk apparatus. In the figure, reference numeral 1 denotes the spindle motor; 1a, a stator (field winding); 1b, a rotator; 2, a flange portion; 3, a hub; 4, a deck; 5 and 6, a magnetic disk (a memory carrier); and 7, a spacer.
The spindle motor 1 is fixed on the deck 4 thorough the flange portion 2. The stator 1a composed of the winding is attached in the flange portion 2. The rotator 1b, or the magnet is disposed rotatively opposite to the stator 1.
The magnetic disks 5 and 6 as the memory carrier are disposed separately from each other on the hub 3. The number of poles of the motor is about from two to four sets, where one set is composed of one N pole and one S pole.
In a case that a conventional rotational speed control method is applied to the spindle motor 1, there is a problem of speed control caused by the error associated with the magnetization of the magnet which constitutes the rotator 1b.
FIG. 11 is a plane view showing an example of the magnetizing state of the magnet which constitutes the rotator 1b. In the figure, symbols N and S denote the magnet of the rotator 1b; and .alpha..sub.1 and .alpha..sub.2 , the angular error associated with the magnetization, and the dot line shows the actual magnetizing state.
In an example shown in FIG. 11, four sets of magnet each composed of one N pole and one S pole are arranged. In an ideal case, the magnets are magnetized as illustrated by the real line, however, the angular errors .alpha..sub.1 and .alpha..sub.2 associated with the magnetization are caused in an actual case because the magnets are magnetized as illustrated by the dot line.
Such a error associated with the magnetization of the rotational magnet of the spindle motor is unavoidable in a manufacturing process.
On the other hand, since the inertia of load is larger in comparison with the torque of the motor in a case of the magnetic disk apparatus, high responsibility is generally not necessary for the servo device for controlling the number of rotations constantly. Therefore, the construction of the servo system can be simplified such that the number of rotations is detected using the detector (for example, a detecting element utilizing Hole effect) for controlling the magnetic pole of the stator (filed winding). In addition, construction elements adapted for high responsibility, for example a sensor for detecting a variation of the magnetic pole of the rotator (magnet), the winding and the like are eliminated. Further, a frequency generator (FG) and the like are not an integral part of the servo system.
FIG. 12 is a plane view illustrating a configuration of Hall-effect elements and the field winding which constitutes the rotator 1a. In the figure, symbols A-C and A'-C' denote the winding; and 8a-8c, the Hall-effect element.
There is provided with three-phase windings A-C and A'-C' in an example shown in FIG. 12. Each pair of the windings A and A', B and B', and C and C' are arranged opposite to each other with respect to the central point. The three hail-effect elements 8a-8c are disposed between the windings A' and B', B' and C', and C' and A respectively to detect the magnetic pole of the rotator.
FIG. 113 shows a timechart diagram representing an example of a timing of exciting the windings of the rotator shown in FIG. 12 and detecting signals generated by the Hall-effect elements. In the figure, symbols Sa-Sc denote the detecting signals of the first to third Hall-effect elements 8a-8c; and A-C denote currents of the three-phase windings.
When the currents flow in the three-phase windings A-C respectively, the motor starts to rotate and the detecting signals Sa-Sc of the Hall-effect elements change as illustrated in the upper part of FIG. 13 according to the rotation of the motor. The timing of currents which are supplied to the windings A-C, that is a exciting pattern is determined in accordance with-states of the detecting signals Sa-Sc of the three Hall-effect elements 8a-8c and the rotational direction. An exciting current is applied to the windings A-C and A'-C' which constitute the rotator according to the exciting pattern.
In a case shown in FIG. 13, the period of each of the detecting signals 8a-8c is equal to the time required foist the rotational angle 90.degree.. That is, if such a simplified detecting means is used, the rotational speed signal is generated according to the number of poles of the rotator (magnet) as shown in FIGS. 12 and 13. Therefore, the signals derived from the detecting means have the same number of periods as the number of sets (poles). The period of the rotational speed signal is in inverse proportion to the rotational speed.
The rotational speed signal includes an error associated with the magnetizing position of the rotator (magnet) as shown in FIG. 11. The quantity of the error is 2.degree.-3.degree. in the unit of angle (one rotation corresponds to the angle 360.degree.) at most. The ratio of the error (the quantity of angle) to the period of the detecting signal increases proportionally according to the number of poles.
FIG. 14 is a timechart diagram representing an example of an one-phase detecting signal detected by a Hall-effect element in the spindle motor including the rotator having an error associated with the magnetization. The horizontal axis represents an angle of rotation, and the vertical axis represents levels of the first to fourth detecting pulses corresponding to the first-fourth cycles respectively.
In an example shown in FIG. 14, a signal with four cycles is detected during one rotation. The sum of the four periods, which are required for the four cycles respectively, is equivalent to the angle of rotation 360.degree. which is constant. However, each of the periods and the ratio of duty are adversely affected by the error. For example, the detecting signal is delayed by the error with respect to the angle 45.degree. during the first cycle corresponding to the first pulse. On the contrary, the detecting signal is put forward with respect to the angle 45.degree. during the second cycle corresponding to the second pulse. Similarly, the detecting signal is delayed or put forward with respect to the angle 90.degree..
In such a conventional speed controlling system by servomechanism using a rotational speed information having an error, an undesirable quantity of control, which is generated by the error associated with the magnetization, causes a ripple of torque in the motor. The result is that the disadvantage such as a very little variation of the rotational speed during one rotation, abnormal sound and the like are revealed, but the total period for one rotation is constant.
On the other hand, it is; impossible to judge the quality of the rotational speed precisely from only the detection of one period of the above-mentioned rotational speed signal under the requirement of 1 and less % accuracy of the rotational speed, that is 1 and less % of the speed regulation.
Since a precise information with respect to the rotational speed is not obtained by the error associated with the magnetization of the magnet which constitutes the rotator in such a conventional spindle motor mentioned above, a shortcoming takes place that a control with such a high degree of accuracy as the 1 and less percentage of the speed regulation can not be performed.
The inventor proposed a servo device and a rotational speed superintendence device for a spindle motor in which a precise rotational speed control can be performed in spite of the error associated with magnetization of the magnet of the rotator in the document of Japanese Application No. 3-146,678.
In these proposed devices, a time required for one rotation is calculated from a rotational speed signal and a control of the rotational speed is performed according to the difference between the calculated time required for one rotation and a target time required for one rotation when the motor rotates at a target number of rotations. The calculated time represents the mean rotational speed during one rotation at the time just before the calculation. Therefore, to be exact the detection of the speed information is delayed with a time corresponding to a half rotation.
The result is that improvement of the control capacity must be prevented by such a trouble as the delay, which corresponds to a half rotation, of the detection of the speed information if the trouble is not eliminated.